Tractors commonly have splined PTO shafts to drive ancillary equipment, such as balers, that are connected the tractor. Such PTO shafts may be provided at the front and/or at the rear of the tractor. Different items of ancillary equipment may require one of two PTO shaft speeds and use couplings with different numbers of splines. To be able to drive all types of ancillary equipment, tractors may include a change speed gearbox to allow the appropriate PTO shaft speed to be selected and may have interchangeable splined coupling shafts.
In addition to the above components, a drive train connecting the PTO shaft to the engine crankshaft also includes a clutch to allow the PTO shaft to be disconnected from the engine. This clutch is disengaged when it is not required to drive the PTO shaft, while changing the speed of the gearbox, while changing the coupling shaft and while connecting ancillary equipment to the tractor.
For safety reasons, it is mandatory to bring the PTO shaft to a complete stop when the clutch is disengaged. As there is a possibility of some frictional drag on the plates of the drive plates of the clutch. it has previously been necessary to use friction brakes to prevent rotation of the PTO shaft when the clutch is disengaged. The entire layout of the drive train of the PTO shaft had therefore to be specially designed to accommodate such a friction brake and an actuation system was required to apply the brake at the same time as disengaging the clutch.